1. Field of the Invention
This invention relates generally to a global position system (GPS) receiver, and more particularly to a portable GPS receiver having an intelligent power supply controller capable of reducing the battery power consumption without position accuracy degradation.
2. Description of the Prior Art
GPS is a positioning and navigation system which receives signals from a plurality of satellites for determining a two or three dimensional position of the receiver. This positioning system is capable of performing a position determination over the entire surface of the globe by receiving signals from a subset of twenty four satellites. These satellites operate on six orbits about 20,200 KM above the earth with each orbit accommodating four satellites. In receiving the signals from these satellites, a GPS receiver periodically computes the latitude, longitude, altitude and time on a real time basis.
To determine a three-dimensional position, the signals from four satellites are required while to make a determination of a two-dimensional position, the signals from three satellites are sufficient. Typical examples of the former are satellites and airplanes and those of the later are ships and cars. However, a three-dimensional position is often required if an automobile is travelling in a mountain region where there are great elevation variations in travelling a short distance.
The GPS system was originally developed for military use. It was then made available for civilian applications including navigation systems for ships, aircraft and automobiles. In the past few years, portable, hand-held GPS receivers have also become publicly available. These convenient portable receivers were made possible, in part, as a result of the miniaturization of electronic devices which continues to reduce the size, weight and power consumption of the electronic components. The portability of a GPS receiver however is often limited by the size and weight of the batteries providing power to the receiver. To sustain prolong periods of operation, a heavy and bulky battery system is required. On the other hand, use of light-weight small batteries require either frequent re-charge or replacement thus making the operation of such a hand-held receiver more expensive and less convenient.
Many U.S. patents disclose power saving methods for digital signal receiving or paging systems. Basically, a receiver or transmitter of these systems is maintained in an "idle" state with very low or complete off power states. Either a hardware or a software system is provided to monitor when incoming signals are received. The receiving system is activated when incoming signals are detected. Then the power is automatically turned off after reception of the signals is complete. Even though this general concept of power saving is widely known, the implemented methods however are not of practical use to a GPS receiver. Unlike the general digital receivers, paging or remote telephone systems, a GPS receiver maintains a table listing all the visible satellites and their positions. A GPS receiver must receive almanac signals from these satellites every twelve hours to compute the satellite positions with moderate accuracy. A more accurate satellite position calculation must also be performed by receiving ephemeris data from the satellites every hour. The collection of almanac data takes about twelve minutes and the collection of ephemeris takes about thirty seconds. A GPS receiver is not totally "idle" even between the signal reckoning times. Continuous status checking of satellites and the data receiving channels must also be performed. Depending on the results of the status checking, a GPS receiver then determines a schedule to sequentially activate each signal receiving channel. Therefore, the method of maintaining an idle state and passively waiting before the arrival of a signal to save battery power as disclosed in the U.S. patents for the general digital signal receiving systems are not useful for reducing the power consumption in a GPS receiver.
Several commercially available GPS receivers are on the market which utilize a set of six AA size alkaline batteries and various means are provided for the receiver operator to save power in preserving the battery life. In SportNav, a Loran C receiver system, a twenty-five hour period of operation is estimated with six AA alkaline batteries. The user is provided the option of a backup battery pack so that the batteries can be quickly replaced. No specific power saving mechanism is implemented in this product.
Another product with the model name PRONAV GPS 100 uses six disposable alkaline batteries and a rechargeable battery pack. It also allows the use of an external power source to provide continuous navigation updates. GPS 100 has a "Battery Saver Mode" operable on a pack of alkaline batteries for fourteen hours and a "QuickFix Mode" which automatically completes four position fixes per hour and allows the receiver to operate for longer periods of time with six alkaline batteries. Under most dynamic circumstances, use of QuickFix to obtain four position fixes per hour is not satisfactory. The usefulness of PRONAV GPS 100 is limited because the length of battery life is likely to be greatly shortened when the limited operations allowable under the "Battery Saver Mode" or "QickFix Mode" are not sufficient to satisfy the position accuracy requirements unless there is external power source readily available.
Another hand-held GPS receiver, the Magellan NAV 1000, is powered by six AA alkaline batteries. For the purpose of reducing battery power consumption and extending the life of the batteries, a PowerSaveR mode is provided under which the receiver can be manually turned on to compute a position fix. After the position fix is stored as the last fix, the receiver then turns itself off. The receiver can also operate continuously and automatically revert to PowerSaveR mode when a `battery low` condition is detected. NAV 1000 also allows the unit to operate on an external power source. It is instructed in the User's manual not to collect almanac information in hand-held operation using the battery because of the concern of the limited battery life. The usefulness of the hand-held GPS receiver would probably be limited due to these limitations.
Therefore, the prior art hand-held portable GPS receivers operating on battery power are typically useful for a very limited period of time if operated continuously. Except where an external power source is readily available, position fix computations on the order of once per second or even once per minute in order to minimize dead reckoning errors would not be possible. This greatly limits the application of hand-held GPS receivers. When no external power source is available, a hand-held battery operated GPS receiver has only limited usefulness due to the short battery life.